The Phorusrhacid ‘Terror Birds’ were large, carnivorous flightless Birds that appear to have held the role of top terrestrial predators in South America throughout much of the Cenozoic. They are known to have colonized parts of North America, during the Great American Biotic Interchange that followed the closure of the Panama Seaway, but subsequently became extinct in both North and South America. Phorusrhacids are also known from the Eocene of Africa, but a number of fossils previously attributed to the group from the Eocene of Europe and North America are now thought to have come from other Birds.
In a paper published in the journal PLoS One on 27 November 2013, Delphine Angst of the Laboratoire de Géologie de Lyon: Terre, Planètes, Environnement at the Département des Sciences de la Terre at Université Lyon, Eric Buffetaut of the Laboratoire de Géologie de l’Ecole Normale Supérieure at the Centre National de la Recherche Scientifique, Christophe Lécuyer of the Laboratoire de Géologie de Lyon: Terre, Planètes, Environnement and the Institut Universitaire de France and Romain Amiot of the Laboratoire de Géologie de Lyon: Terre, Planètes, Environnement, redescribe two sets of Bird remains from France and Switzerland formerly assigned to the Gastornithids (unrelated large, flightless, carnivorous Birds) and Ratites (the group of flightless Birds that includes Ostriches, Emus and Kiwis) as Phorusrhacid ‘Terror Birds’.
The first set of remains come from a fissure filling at Lissieu, north of Lyon, dated to 43.5 to 41.2 million years ago, or Middle Eocene, which has subsequently been destroyed by quarrying. This comprises a set of foot bones and claws originally described as Diatryma? cotei in 1936. Diatryma is now considered to be the same species as Gastornis, and under either name is quite unlike the Lissieu specimens, having hoof-like claws.
Material from Lissieu. L71: distal end of left tarsometatarsus, (A) cranial view, (B) caudal view; L72, FSL337282: left median (III) trochleae in lateral view; L68, L69, L70, L73, L135, L470, L468: ungual phalanges; L66, L67: phalanges. L66, L67, L68, L69, L70, L71, L72, and L73: type series described by Gaillard (1937) kept in the Musée des Confluences, Lyon. FSL337282: supplementary material kept in the University Lyon, Lyon L135, L470, and L468: supplementary material kept in the Musée des Confluences, Lyon. All scale bars are 1 cm. Angst et al. (2013).
The second set of remains come from a fissure at Egerkingen in the canton of Solothurn in Switzerland, and were described as Eleutherornis helveticus, which was thought to be a type of Ratite in 1940. This material comprises parts of the pelvis plus more foot material, and are thought to date from about the same time as the French material.
Material from Egerkingen. Eh.781: anterior part of a pelvis (synsacrum and ilia), (A) ventral view, (B) dorsal view, (C) cranial view; Eh.782: posterior part of a synsacrum, (A) ventral view, (B) dorsal view. Ef. 998, Ef. 999, Ef. 1000: phalanges; Ef. 1001 and unnumbered: ungual phalanges. Scale bars are 1 cm. Angst et al. (2013).
Angst et al. conclude that all these remains are likely to have come from one species of Bird, and that that that Bird was a Phorusrhacid, which they identify as Eleutherornis cotei (the French material was described first, under the name Diatryma? cotei; ‘Diatryma’ is now considered invalid, but ‘cotei’ is still valid, as is ‘Eleutherornis’, a generic name created to describe the Swiss material).
The presence of a Phorusrhacid Bird in Europe in the Eocene requires some explanation, as Europe has a fairly good Avian fossil record during the Tertiary, and other Phorusrhacids are not known, implying that the group were not long-term European residents, but briefly invaded Europe during the Middle Eocene. A dispersal directly from South America to Europe is unlikely as the two continents are not close, nor is a dispersal via North America likely, as North America also has a relatively good Avian fossil record and no Phorusrhacids. This makes a dispersal to Europe via Africa the most likely explanation, as Phorusrhacids are known to have been present in Africa, although the poor Avian fossil record in Africa makes it hard to assess how prevalent they were.
The appearance of the Phorusrhacids is thought to have considerably post-dated the separation of the African and South American continents in the Early Cretaceous, suggesting that the group was able at some point to cross the Atlantic. This at first seems an unlikely proposition, however in the early Tertiary the ocean was considerably narrower than it is now, and is thought to have had extensive ranges of volcanic islands along the Walvis Ridge and Rio Grande Rise, and several other groups are thought to have made this crossing (predominantly from Africa to South America, although this has no bearing on the distribution of Phorusrhacids into Europe, and it is uncertain whether the group first arose in Africa or South America).
Similarly, while Africa and Europe were still yet to collide, the Tethys Ocean was closing rapidly during the Eocene, and the area where the Mediterranean now lies was dotted with volcanic islands, as well as at least two smaller continental blocks, the Alboran and Apulian, making such a crossing possible, and once again the Phorusrhacids are not the first group for which this such a dispersal path has been suggested.
Paleobiogeographical distribution of the Phorusrhacidae in the Eocene. Arrows: likely transoceanic dispersal routes of the Phorusrhacidae. Angst et al. (2013).
As well as the appearance of Phorusrhacids in Europe in the Middle Eocene, their subsequent disappearance from Europe requires an explanation. Tertiary African ecosystems are not well understood, but those of South America are, and the Phorusrhacids appear to have been the top predators in many South American ecosystems for much of the period. However South America lacked predatory Mammals during this time, the other predators with which Phorusrhacids shared the continent being Marsupials. Placental Mammal predators (Carnivora and later Homo sapiens) reached South America during the Great American Biotic Interchange, which also allowed the Phorusrhacids to briefly colonise North America, an event that was shortly followed by the extinction of both the Phorusrhacids and large Marsupial predators in the Americas (large Marsupial predators also became extinct in Australia shortly after the arrival of Placental predators, in this case Homo sapiens and Canis lupus). In Europe the Phorusrhacids would have encountered two well established groups of carnivorous Placental Mammals, the Carnivora and (now also extinct) Creodonta, which may explain their inability to become established as a permanent part of the fauna.
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